ABSTRACT Tick-borne relapsing fever (TBRF) is one of the Neglected Tropical Diseases. But TBRF's distribution extends beyond the tropical regions to most of the rest of the world. Sylvan life cycles, comprising tick vectors and mammalian reservoirs, also exist in several areas in North America. Most human cases of relapsing fever in North America are from Borrelia hermsii, whose endemic foci mainly are coniferous forests in mountainous areas. In comparison to the related agents of Lyme disease, little is known about the population biology and genetics of TBRF agents. The conceptual entry point for this 2-year, small grant project is the null hypothesis is that these two groups of tick-borne spirochetes have similar population structures, namely, multiple strains, with the predominant strain a plurality, not a majority. Investigations carried out on the collected B. hermsii will also advance understanding of the evolution of antigenic variation of multiple, highly-polymorphic proteins (VMPs). Other expected benefits include estimates of effective population size of B. hermsii, characterization of the population genetics of the tick vector Ornithodoros hermsi and its Coxiella sp. endosymbiont, and survey of rodents for infection with B. hermsii. The 2-year small grant project combines field work and laboratory studies. It leverages the experience and resources of the PI's laboratory with the field expertise and resources of the study's collaborators. Field studies will be at two sites of the University of California's Natural Reserve System. These sites represent isolated mountain niches, or sky islands. At the sites, ticks will be systematically collected, using an innovative approach, from burrows and nests of chipmunks and squirrels. Personnel of the California Department of Public Health will trap and bleed rodents as part of their routine surveillance for zoonoses. The laboratory studies entail (i) polymerase chain reaction (PCR) and multilocus sequence typing (MLST) of DNA extracted from ticks, as well as samples of rodent blood provided by CDPH, (ii) serology of rodent blood, (iii) genome sequencing of selected isolates, and (iv) phylogenetic and population genetic analysis. Aim 1: Population structure of Borrelia hermsii at two ?sky island? sites in California with recent human cases of relapsing fever. Systematic collection of O. hermsi ticks from field sites in the White-Inyo Mountains and Sierra Nevada Mountains, followed by (a) determination of the prevalence of B. hermsii in the ticks by PCR and by injection of laboratory mice and (b) serology for evidence of present or past infection of wild rodents. Identified B. hermsii in ticks or rodents will be genotypes by MLST. Aim 2: Population genetics of B. hermsii and evolution of its VMP genes. Genome sequencing of B. hermsii, genotyping of O. hermsi ticks, and genotyping of the Coxiella sp. endosymbiont. Estimates of effective population size, analysis for evidence of selection and recombination in VMP genes, studies of the population structures of O. hermsi ticks and the Coxiella sp. endosymbiont, and initial estimates of rates of evolution.